The standard RF generator used in the majority of electron beam printers today uses the natural resonance of the driver and the print cartridge. U.S. Pat. No. 4,841,313 (the disclosure of which is hereby incorporated by reference herein), describes the purpose of the generator and a transformer-coupled resonant circuit, wherein the inductance of the secondary winding and the capacitance of the print cartridge load define the resonant frequency. It also describes the method of AC generation, oscillator feedback, and trigger generation that's responsive to the secondary AC. U.S. Pat. No. 5,142,248 (the disclosure of which is hereby incorporated by reference herein), expands on the '313 patent by the addition of a common feedback circuit connected to all transformers and the enable circuit.
Through all the descriptions in the above references, nothing is disclosed about the print cartridges power consumption or the need for an efficient RF generator or transformer for the generator.
Current practices that the present invention is intended to improve upon include:
1. Printer speeds limited to less than 200 FPM for an 18 inch 600 DPI print cartridge. PA1 2. Print gradients on current 18" print cartridge. PA1 3. Ceiling of 2.5 MHz RF frequency for efficient operation. PA1 4. Limited transformer turns ratio (due to ferrite properties) PA1 5. Limited choice of operating frequency (due to turns ratio) PA1 6. Transformer failures due to temperature. PA1 7. Upper frequency limited by core temperature to a compromised limit of 5 MHz. PA1 8. High power losses in transformer drive transistor. PA1 1. Printer speeds to 300 FPM (e.g. about 210-300 FPM and all narrower ranges within that broad range) for an 18 inch 600 DPI print cartridge. PA1 2. No print gradient due to an over driven transformer. PA1 3. Efficient operation at 5 MHz is now possible. PA1 4. Greater flexibility in transformer turns ratio designs. PA1 5. Choice of operating frequency due to the low loss properties of the powdered iron core. PA1 6. No transformer temperature failures due to core loss. PA1 7. No compromise operation at 5 MHz, with higher frequency designs possible by proper core selection. PA1 8. Reduced power losses in the transformer drive transistor.
Conventional RF generators used to drive a 600 DPI 18 inch print cartridge use 2000-2200 volt, 5 MHz resonant transformers operating at power levels of 550 watts peak. Core loss at the frequencies required to drive the 600 DPI cartridge have been found to raise the temperature of the transformer to 100 degrees C. when running equivalent press speeds of 150 feet per minute, the temperature quoted by core manufactures as a recommended upper limit. Core temperature continues to rise as the printers speed rises. One manufacture disables its generator when printing and no charge is required, thus lowering the core temperature and allowing for greater printer speed. A method of disabling a generator while printing is described in U.S. Pat. No. 4,990,942. This method reduces the burden on the RF generators as well as the transformers temperature. However printing speeds then become dependent on coverage.
To get full output, the current designs over drive the transformer. This puts undue stress on the supporting drive electronics and causes the output wave to be distorted. The distortion can result in a print density gradient across the length of the print cartridges' RF driveline. It also tends to reduce the natural resonant frequency of the generator.
Conventional transformers have been constructed with ferrite material. The advantage of this type of construction is that the core permeability can be chosen to ensure a minimum number of wire turns for a given value of inductance, thus reducing resistive and capacitance loses. Above 2.5 MHz the advantages of low wire resistance are over shadowed by the ferrite core losses for this type of application.
In the energy efficient HF RF oscillator for electron beam printers according to the invention, the above deficiencies are overcome by using a "powdered iron core transformer" and a "dual pulse width generator", with the following results:
The remainder of the system according to the invention utilizes standard digital, RF and analog practices found in gated power oscillators.
The RF Generators according to the invention produce bursts of high voltage AC, which are applied to the drive lines of a print cartridge, causing an ion producing discharge. Typically there are ten generators on each of two identical boards to drive the nineteen lines on the cartridge. When the board is used to drive the right side of the cartridge, it utilizes all ten channels. When used as a left driver, only nine channels are typically utilized.
A resonant feedback oscillator circuit, using digital logic elements as part of a nonlinear feedback path, produces the high voltage AC burst. The oscillator approach was chosen over an amplifier because it offers greater stability in the presence of a changeable load and an opportunity to achieve higher efficiency. This is desirable because of the high power levels of 550 watts peak at 5-MHz are involved. Voltages of 2200 volts peak to peak require a step up transformer.
According to one aspect of the present invention an electron beam printer assembly is provided comprising: An RF generator including a powdered iron core transformer, a power driver, control logic, and an oscillator feedback circuit; and, the transformer connected to an electron beam print cartridge. Preferably the RF generator further comprises a dual pulse width generator operatively connected to the transformer. The transformer preferably comprises primary, secondary, and clamping windings, the secondary winding connected to the cartridge. The cartridge (preferably a conventional cartridge such as shown in U.S. Pat. Nos. 4,160,257, 4,408,214, 4,494,129, 5,014,076 and/or 5,315,324) may comprise an eighteen inch 600 DPI print cartridge. Typically the assembly is capable of effectively printing a substrate moving at a speed over 200 feet per minute, e.g. 210-300 feet per minute.
The dual pulse width generator may comprise a NOR gate, capacitor, two resistors, and a diode, all connected to an adjustable voltage source. The transformer may have a toroid-shaped core, e.g. of carbonyl SF. Typically ten of the generators are mounted to a drive board, and two drive boards are provided; and the cartridge has nineteen channels.
According to another aspect of the present invention an electron beam printer assembly is provided comprising: An RF generator including a dual pulse width generator connected to a transformer; and an electron beam print cartridge connected to the transformer. The details of the components are preferably as described above.
According to another aspect of the present invention a method of imaging a substrate (such as paper) using an electron beam print cartridge and an RF generator is provided. The method comprises: (a) Moving the substrate at a speed greater than 200 FPM. And, (b) while practicing (a), operating the RF generator at a frequency greater than 3 MHz to drive the cartridge and thereby image the moving web.
In the method, (b) is preferably practiced using an RF generator having a powdered iron core transformer, and a dual pulse width generator. Also (b) is typically practiced using a 600 DPI eighteen inch print cartridge, and operating the RF generator at a frequency of about 5 MHz. In the practice of the method, (a) is preferably practiced at a speed of about 210-300 FPM.
It is the primary object of the present invention to provide enhanced efficiency of an RF generator for powering a print cartridge. This and other objects of the invention will become clear from an inspection of the detailed description of the invention and from the appended claims.